I-94 project bringing new Stadium Interchange. It includes driving on a road's opposite side

Interstate 94's expansion on Milwaukee's west side will feature something many area drivers likely have never encountered: an interchange which involves driving briefly on a road's opposite side.

Known as a diverging diamond interchange, it's a key part of the Wisconsin Department of Transportation's plans for widening I-94 from four lanes to six lanes between 16th and 70th streets. That $1.74 billion project, recently approved by the Federal Highway Administration, is to begin construction in late 2025 and run until 2031.

The diverging diamond will save up to $90 million on the project and use less space. It also will likely bring more crashes by drivers entering the interchange − even as studies show such designs generally improve safety.

Here's what to know.

How does a diverging diamond operate?

The interchange's I-94 left-turn on-ramps will be accessed through a traffic pattern with cars driving briefly on the opposite side of the Stadium Freeway, in a weaving pattern which uses traffic signals.

That same weaving pattern is used by drivers staying on the Stadium Freeway as they pass through the interchange.

The interchange includes right-turn on-ramps that aren't accessed through the weaving pattern.

Tips for navigating the interchange

According to WisDOT's website, drivers "follow signs, signals and pavement markings to cross through the intersection at the first set of traffic lights. Traffic appears as if on a one-way street."

"All left turns onto the freeway are free flow, meaning vehicles do not have to stop to access the ramp.

"Vehicles going straight simply proceed through a second set of traffic lights," the website said.

Diverging diamond is smaller, less costly

Such interchanges are smaller and less expensive than traditional diamond interchanges − saving the I-94 project $70 million to $90 million, according to a WisDOT document submitted to the Federal Highway Administration.

The department's Office of Public Affairs said no one was available for the Milwaukee Journal Sentinel to interview about the diverging diamond.

In a written response, the department said the public wants the Stadium Interchange properly sized "to fit in a constrained urban corridor."

That's an important factor, said Robert Schneider, a professor and co-chair of University of Wisconsin-Milwaukee's Department of Urban Planning.

The interchange is near American Family Field − where part of the parking lots will be studied for possible commercial development, he said. There also are opportunities to connect the new interchange with nearby biking and walking trails.

"It's very valuable space," Schneider said.

Meanwhile, a portion of the ballpark's parking lots will be needed to accommodate the new interchange, which will shift east.

Planned interchange means more crashes over current system

Typically, a diverging diamond improves safety by eliminating the need for drivers on a surface street to make left turns against oncoming traffic while accessing a freeway's ramps.

But the Stadium Interchange doesn't involve a surface street. It's a "system interchange" which serves two freeways. So, drivers currently don't make left turns against oncoming traffic.

Crash rates are projected to be higher with the diverging diamond compared with either a hybrid interchange − which would be larger and higher − or maintaining the current interchange, known as the "no build" alternative.

The hybrid would have 80% to 90% more crashes, and the diverging diamond would have about 150% more crashes than the no-build option, WisDOT says in its document outlining the I-94 expansion plans.

"The No-build alternative is safer because the Stadium Interchange would remain a system interchange. With a system interchange, there are no intersections where vehicles turn in front of other vehicles, and therefore, there is less potential for crashes," according to the WisDOT document.

"The hybrid interchange would have fewer conflict points than the diverging diamond interchange, thus it is logical that a hybrid interchange would be predicted to be safer than a diverging diamond interchange.

"While the diverging diamond interchange is predicted to have slightly less fatal crashes than the hybrid interchange, the diverging diamond interchange would have more injury and property-damage-only crashes," the document said.

WisDOT cites trade-off between safety, costs

The WisDOT document said "safety was just one element" in its decision to select a diverging diamond.

"Both the diverging diamond interchange and the hybrid interchange would be safe and suitable for their intended use of moving traffic in a constrained urban corridor. The increase in crashes is a trade-off of providing a lower-level interchange type" which serves a surface street − known as a "service interchange."

A diverging diamond "is one of the safest and most efficient types of service interchanges" and it meets the goal of downsizing the Stadium Interchange, WisDOT said.

That latter point reflects the Stadium Freeway's underused condition. Indeed, WisDOT is studying the possibility of converting a portion of the freeway, between Wisconsin and Lisbon avenues, to a surface street.

"From a safety perspective, the Stadium Interchange selection includes a comparison based on a benefit-cost ratio, which follows WisDOT policy," the WisDOT document said.

"While the system interchange (No-build alternative) is associated with the fewest crashes, if the Stadium Interchange were to be reconstructed as a system interchange, it would cost at least $100 million more than the diverging diamond interchange.

"The hybrid interchange has fewer crashes but costs approximately $90 million more than the diverging diamond interchange.

"Combining the construction costs and safety benefit costs, a rebuilt system interchange and hybrid interchange result in a benefit-cost ratio of less than 1.012 compared to the diverging diamond interchange.

"These results show that there is not a sufficient safety benefit of the hybrid interchange or a rebuilt system interchange to justify the additional costs," the WisDOT document said.

Studies show diverging diamonds improve safety

Diverging diamonds generally improve safety for freeway interchanges which serve surface streets, according to various studies.

A 2016 study of crash data by a Utah State graduate engineering student at five Utah freeway locations which switched from traditional diamond interchanges to diverging diamonds found "positive safety impacts at most study locations" − particularly with a decline in crashes that cause injuries and fatalities.

Diverging diamonds reduced crashes by an average of almost 40% after being constructed in 26 places throughout the United States, according to research published in 2019 by the Transportation Research Board’s National Cooperative Highway Research Program. The rates of injury and fatal crashes, on average, fell by over 50%.

Also, a study of Missouri diverging diamonds found they reduced the number and severity of crashes.

However, that 2015 study, led by a University of Missouri engineering professor, also found 4.8% of all fatal and injury crashes occurring at the ramp terminal of a diverging diamond were wrong-way crashes.

Wisconsin has other diverging diamonds

Wisconsin's other diverging diamond interchanges, according to WisDOT's website, are located in Appleton, at Wisconsin Highway 441/Oneida Street; Beloit, at I-39/90 and Wisconsin Highway 81, and in the Janesville area, at I-39/90 and Wisconsin Highway 26/Milton Avenue and I-39/90 and Wisconsin Highway 11/Avalon Road.

Those interchanges have been built since 2016, and WisDOT says it's seen reductions in fatal and serious injury crashes compared to the interchanges they replaced.

In the Milwaukee area, a diverging diamond is being built at I-43 and Brown Deer Road.

Tom Daykin can be emailed at tdaykin@jrn.com and followed on Instagram, X and Facebook.

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This article originally appeared on Milwaukee Journal Sentinel: Milwaukee's I-94 project to feature diverging diamond interchange